carrier class availability prediction for hybrid fso/rf system … · · 2015-02-13the...

Abstract—Availability is considered as the main parameter of

evaluating a Hybrid FSO/RF link quality. An accurate carrier

class availability prediction of Hybrid FSO/RF is needed. In

tropical regions, among different weather influences, rain plays

a major role. Precipitation decrees are available for frequencies

above 10 GHz. In this paper we will analyze the effect of rain on

Hybrid FSO/RF to find the optimum operating frequency to

back-up the FSO link besides tradeoff between speed of RF and

link availability of FSO in tropical environment; we will also

provide better prediction of link availability. ITU-R specific

rain attenuation of FSO and RF models has been used for the

analysis. From the results, a 30 GHz and below are suggested to

be used as a back-up of FSO link to achieve carrier grade

availability under the impact of rain.

Index Terms—Carrier grade availability, heavy rain

attenuation hybrid FSO/RF, ITU-R specific rain attenuation

models.

I. INTRODUCTION

Free Space Optics is a wireless technology which uses

laser as a medium of transmission between transmitter and

receiver. This technology is a line of sight (LOS) where

transmitter and receiver should be on mountain or at the roof

top of buildings.

FSO has many advantages such as high bandwidth, easy to

install where you can install transmitter and receiver beside a

window and within days, free licensing of frequency, cost

effectiveness where no digging is needed and very high

security where the signal cannot be hacked.

The big challenge facing this technology and considered a

disadvantage is that FSO especially in long distances can

easily be affected by weather conditions like fog, haze, rain

and scintillation [1], [2]. In temperate regions, fog is consider

the main factor which influences the performance of FSO

link availability [3], [4]; whereas in tropical regions heavy

rainfall rate is expected to be the limiting factor for FSO link

availability. These weather conditions limit the distance of

FSO and decrease the link availability [5]. In order to

compensate this problem we need to have a comprehensive

FSO system; one of the solutions is to implement FSO link

with a back-up RF (Hybrid FSO/RF system). FSO/RF system

can enhance the performance of FSO system and optimize the

link quality such as the availability of FSO performance; by

using FSO/RF the availability can increase from 99.99%

Manuscript received March 9, 2014; revised June 1, 2014.

The authors are with the Department of Electrical and Computer

Engineering at IIUM, 53100 KL, Malaysia (e-mail: [email protected], [email protected]).

(enterprise class) to 99.999% (carrier class) [2]. In this paper,

the availability is calculated using link budget of FSO system

and microwave. As the rain attenuation of Hybrid FSO/RF is

independent of wavelength, the analysis will be based on

rainfall rate [6]. ITU-R specific rain attenuation models for

FSO and RF are used for the analysis. The effect of rain

attenuation on the availability of Hybrid FSO/RF is analyzed

by examining the distance of different RF frequencies. A

recommendation of optimum operating frequency for

tradeoff of FSO availability is suggested.

Over the years, FSO technology has gained acceptance in

telecommunication industry mostly in enterprise campus

network. This paper provides recommendations to local

telecom service provider about possible availability figures

of carrier and enterprise class that can be useful for the

deployment of Hybrid FSO/RF link as the last mile solution,

back-up for fiber optic and other applications. Fig. 1 below

shows the proposed approach of the link availability

prediction used in this paper.

Fig. 1. Flow chart of proposed approach.

II. FSO LINK BUDGET

A. Geometric Loss

Light beam is expanding as it travels from point of origin

outwards. The beam usually adopts a cone shape and a

Ahmed Basahel and Wajdi Al-Khateeb

Carrier Class Availability Prediction for Hybrid FSO/RF

System in Heavy Rainfall Regions Based on ITU-R

Models

International Journal of Computer and Communication Engineering, Vol. 3, No. 6, November 2014

404DOI: 10.7763/IJCCE.2014.V3.358

divergence angle determines how much the beam spreads as

it travels through air. Thus, this will lead to the fact that a big

portion of the light will miss the receiver aperture at the

receiver side and will be considered as a loss. This loss is

referred to as a geometric loss or attenuation. Geometric

attenuation has three main parameters which can help in

optimizing the performance of FSO link which are

minimizing divergence angle and link distance, increasing

the receiver aperture area. The divergence angle and receiver

aperture area are related to the design of FSO.

Mathematically, geometrical attenuation can simply be

derived from Fig. 2 below. The portion of light which misses

the receiver aperture is shown.

Fig. 2. Propagation of the light beam.

GeomAtt =

So, simply by dividing these two circle areas as:

= (

)

(

) =

= (

)

(1)

The geometric attenuation can be in dB as follows:

GeomAtt (dB) = 10 (

)

GeomAtt (dB) = 20

(2)

B. Rain Attenuation

In regions with heavy rainfall rates such as tropical areas,

rain is the main atmospheric attenuation parameter. One of

the models which estimates attenuation due to rain for FSO

links in tropical areas is:

Ap = W + A1km × L × r1 + 2 (dB) (3)

Ap is the overall rain link attenuation for % of the year for

path length, L, W, loss due to water on the FSO transceiver

window, A1km, the rain path attenuation, r1, normalized

reduction factor. The additional 2 dB is taking into account

the smoke haze and scintillation effects. The above model is

an experimental model proposed in Singapore by [7].

C. FSO Link Margin

The concept of link margin of FSO link can be shown

clearly in Fig. 3. The condition of link availability of Hybrid

FSO/RF is LM ≥ additional loss. Also, from this condition the

maximum distance of Hybrid FSO/RF can be achieved when

link margin = 0.

Fig. 3. An example of FSO link margin diagram.

III. MICROWAVE LINK BUDGET

A. Free Space Loss

The total transmission loss of a microwave link can be

calculated using the following formula [8].

AttenuationdB = 92.45 + 20 log FGHz + 20 log Dkm + e dB (4)

e is excess attenuation (dB) due to rainfall.

B. Rain Attenuation

Again the additional attenuation in microwave will be due

to rain. The ITU-R specific attenuation model for rain of a

microwave line of sight is used to determine the attenuation

due to rain [9].

IV. LINK AVAILABILITY OF HYBRID FSO/RF

The estimate availability figure calculated based on

existing commercial Hybrid FSO/RF parameters is shown in

Table I. In (3), ITU-R Japan model (A1km = 1.58R0.63

) is

employed for FSO channel to calculate A1km parameter [10].

The reason we use this model is because of the measurement

conducted to develop this model up to 80 to 90 mm/hr rain

rate which gives close figure of rain rate in heavy rainfall

regions. To calculate the fade margin as a function of distance

we used the rain rate of region N of Table I. in ITU-R

PN.837-1 with corresponding percentage of time [11]; and

specific rain attenuation model of RF. The simulations have

been done for many different frequencies to find the optimum

operating frequency to back-up FSO link. Fig. 4 and Fig. 5

show the fade margin as a function of distance of Hybrid FSO

with lower back-up frequencies 10 & 25 GHz. 10 GHz gives

carrier class availability for distance more than 5km. 25 GHz

gives carrier class availability for a maximum up to 800m

distance.

A. FSO/10 & 25 GHz

The reason 10 GHz can go up to a long distance is due to

the fact that frequencies below 10 GHz are not affected only

by rain but all weather conditions, whereas above 10 GHz are

attenuated due to rainfall and atmospheric absorption. In high

frequencies the wavelength will be shorter than the raindrop

size causing absorption and scattering.

TABLE I: HYBRID FSO/RF SYSTEMS PARAMETERS USED IN CALCULATIONS

Channels Transmit

Power Sensitivity

Divergence ang.

/ Antenna Gain

Receiver Ape.

/ System loss

FSO 28 dBm -34 dBm 2.7 mrad 0.2 m

RF 26 dBm -96 dBm 22 dB 2 dB

Area of the beam at

a distance (L)

Beam divergence

Link length L

Transmitter

θ

Area of receiver capture

with diameter (D)

10

PrGeoatt

Pr0 = Pr rain atten+ PrGeoatt

LM = Pr0 - sensitivity

Atm. loss due to rain = Ap = W + A1km L r1 + 2

Geoatt = 20 log D / L θ

FSO Tx FSO RxFSO Channel

-35

-20

Thr = - 46 0.5 km FSO link

available at

Max Distance

2 km 3 km

11 dBm Margin

Op

tica

l P

ow

er (

dB

m)


405

Fig. 4. Fade margin as a function of distance of Hybrid FSO/10 GHz.



B. FSO/ 30 GHz

Fig. 6 shows FSO link with the optimum choice of 30 GHz

back-up. 30 GHz can provide carrier class availability over a

distance of almost 750m only.

It can also provide long distance if the availability is not

the priority.

C. FSO/ 40 & 60 GHz

Fig. 7 and Fig. 8 show the fade margin as a function of

distance of Hybrid FSO with higher frequencies of 40 GHz

and 60 GHz. These two frequencies can operate within only a

few hundred meters for carrier class availability.

Enterprise class availability for 10, 25, 30, 40 & 60 GHz

back-up can operate over long distances as shown in Fig. 9,

whereas in Fig. 10 carrier class availability with good enough

resolution for Hybrid FSO/RF is shown. In the case of FSO

outage, the transition or switching can be done at 30 GHz

back-up to provide carrier class availability with

consideration of data rate.



Fig. 9. Enterprise class availability of Hybrid FSO/RF.

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8 9 10

Fad

e M

argin

(d

B)

Link Distance (km)

99.999% FSO

99.997% FSO

99.99 % FSO

99.97% FSO

99.9% FSO

99.7% FSO

99% FSO

99.999% 10 GHz

99.997% 10 GHz

99.99% 10 GHz

99.97% 10 GHz

99.9% 10 GHz

99.7% 10 GHz

99% 10 GHz

10 GHz System Margin

FSO System Margin

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8

Fa

de M

arg

in (

dB

)

Link Distance (km)

99.999% FSO

99.997% FSO

99.99% FSO

99.97% FSO

99.9% FSO

99.7% FSO

99% FSO

99.999% 25 GHz

99.997% 25 GHz

99.99% 25 GHz

99.97% 25 GHz

99.9% 25 GHz

99.7% 25 GHz

99% 25 GHz


FSO System Margin

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7 8

Fad

e M

argin

(d

B)

Link Distance (km)

99.999% FSO

99.997% FSO

99.99% FSO

99.97% FSO

99.9% FSO

99.7% FSO

99% FSO

99.999% 30 GHz

99.997% 30 GHz

99.99% 30 GHz

99.97% 30 GHz

99.9% 30 GHz

99.7% 30 GHz

99% 30 GHz


FSO System Margin

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7

Fad

e M

argin

(d

B)

Link Distance (km)

99.999% FSO

99.997% FSO

99.99 % FSO

99.97% FSO

99.9% FSO

99.7% FSO

99% FSO

99.999% 40 GHz

99.997% 40 GHz

99.99% 40 GHz

99.97% 40 GHz

99.9% 40 GHz

99.7% 40 GHz

99% 40 GHz


FSO System Margin

0

5

10

15

20

25

30

35

40

0 1 2 3 4 5 6 7

Fad

e M

argin

(d

B)

Link Distance (km)

99.999% FSO

99.997% FSO

99.99 % FSO

99.97% FSO

99.9% FSO

99.7% FSO

99% FSO

99.999% 60 GHz

99.997% 60 GHz

99.99% 60 GHz

99.97% 60 GHz

99.9% 60 GHz

99.7% 60 GHz

99% 60 GHz


FSO System Margin


406

Fig. 10. Carrier class availability of Hybrid FSO/RF.

V. CONCLUSION

Availability in general depends on the internal parameter

of Hybrid FSO/RF system such as divergence angle, receiver

aperture of FSO and antenna gain of RF. Also, it depends on

the weather in particular. To achieve carrier class availability

of Hybrid FSO/RF under the impact of heavy rainfall climate

in tropical regions, a 30 GHz is considered the optimum

operating frequency to tradeoff between speed (data rate) of

RF and availability of FSO at a distance of almost 750m only.

In the case of enterprise class availability or data rate is not

the priority; frequencies below 30 GHz are recommended to

be used for long distances (few kilometers).

REFERENCES

[1] O. Bouchet, H. Sizun, and C. Boisrobert, Free Space Optics:

Propagation and Communication, 1st ed., London, UK: ISTE Ltd,

2006. [2] I. I. Kim and E. Korevaar, “Availability of free space optics (FSO) and

hybrid FSO/RF systems,” SPIE. Optical Wireless Communications IV,

vol. 4530, pp. 84-95, Nov 2001. [3] F. Nadeem et al., “Weather effects on hybrid FSO/RF communication

link,” IEEE J. Sel. Area Commun., vol. 27, no. 9, pp. 1687-1697,

December 2009. [4] F. Nadeem, E. Letigeb, and O. Koudelka, “Comparing the rain effects

on hybrid network using optical wireless and GHz Links,” in Proc.

International Conference on Emerging Technologies, October 18-19,

2008, pp. 156-161.

[5] A. Prokes. (June 2009). Atmospheric effects on availability of free space optics systems. Optical Engineering. [Online]. 48(6). Available:

http://spie.org/x648.html?product_id=839757.

[6] M. Achour. (April 2002). Simulating atmospheric free-space optical propagation: Part I, rainfall attenuation. Proceedings of the SPIE.

[Online]. 4635. Available:

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=873412

[7] S. Rao, J. Ong, K. Timothy, and D. Venugopal. (June 2013).

Propagation of free space optical links in Singapore. Indian Journal of Radio & Space Physics. [Online]. 42. pp. 182-186. Available:

http://nopr.niscair.res.in/bitstream/123456789/19412/1/IJRSP%2042

%283%29%20182-186.pdf. [8] R. Freeman, Radio System Design for Telecommunications, 3rd ed.

New Jersey, US: Wiley, 2007, ch. 9, p. 464.

[9] Rec. ITU-R P.838-3, Specific Attenuation Model for Rain for Use in Prediction Methods, International Telecommunication Union, 2005.

[10] Rec. ITU-R P.1814, Prediction Methods Required for the Design of

Terrestrial Free-Space Optical Links, International Telecommunication Union, 2007.

[11] Rec. ITU-R P.837-1, Characteristics of Precipitation for Propagation

Modeling, International Telecommunication Union, 1994.

Ahmed Abdullah Basahel received his M.Sc. degree from University of Malaya (UM) 2010. He is currently

a full-time PhD student at the Department of Electrical

and Computer Engineering (IIUM). His research area is availability of hybrid free space optics / microwave.

Wajdi Fawzi Al-Khateeb received his PhD degree

from the International Islamic University Malaysia (IIUM), Malaysia 2006 and his M.Sc. degree from the

Technical University of Berlin, Germany in 1968. His

research interest is mainly in the reliability engineering, fault tolerant systems, QoS networking,

free space optics and microwave radio links. He is currently an associate professor at the Department of

Electrical and Computer Engineering, IIUM. Besides

his academic activity, he was appointed as the leader of consultancy team to plan, design and supervises the ICT infrastructure project with more than 30

thousand data/voice nodes to support the ICT applications of the university.


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carrier class availability prediction for hybrid fso/rf system … · · 2015-02-13the...

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